The heart is the center of a person's circulatory system. It includes an electro-mechanical system performing two major pumping functions. The left portions of the heart draw oxygenated blood from the lungs and pump it to the organs of the body to provide the organs with their metabolic needs for oxygen. The right portions of the heart draw deoxygenated blood from the body organs and pump it to the lungs where the blood gets oxygenated. These pumping functions are accomplished by cyclic contractions of the myocardium (heart muscles). In a normal heart, the sinoatrial node generates electrical impulses called action potentials at a normal sinus rate. The electrical impulses propagate through an electrical conduction system to various regions of the heart to excite the myocardial tissues of these regions. Coordinated delays in the propagations of the action potentials in a normal electrical conduction system cause the various portions of the heart to contract in synchrony to result in efficient pumping functions indicated by a normal hemodynamic performance. A blocked or otherwise abnormal electrical conduction and/or deteriorated myocardial tissue cause dysynchronous contraction of the heart, resulting in poor hemodynamic performance, including a diminished blood supply to the heart and the rest of the body. The condition where the heart fails to pump enough blood to meet the body's metabolic needs is known as heart failure.
Electrical stimulation therapies have been applied to restore functions of the electrical conduction system and reduce the deterioration of myocardial tissue. Their potential benefits to a patient are achieved or maximized when such therapies are adaptive to the patient's cardiac conditions and metabolic needs, both changing over time. In one example, delivering pacing pulses at a relatively high rate may satisfy the patient's instantaneous metabolic need for participating in an intense physical activity but result in further deterioration of myocardial tissue. In another example, an electrical therapy preventing further deterioration of myocardial tissue may significantly limit the patient's exercise capacity when the therapy is being delivered.
For these and other reasons, there is a need to modulate the delivery of cardiac electrical therapies based on the patient's changing needs and conditions.